One thought that just crossed my mind is this:
We could write a code generator that generates the appropriate C# stubs for our built-ins. The generator would generate a default stub based on the arity of the original Ruby method (our real implementations rely on our binder to locate the correct strongly typed method). That method would be marked with something like a NotImplementedAttribute, and throw a NotImplementedException by default.
This way we could run a tool over the library assembly to report progress (or an estimate of progress). We could also introduce a NotCompletedAttribute to indicate methods that are a work-in-progress.
Thoughts?
-John